The present invention relates to an apparatus for producing book blocks from a stack of signatures that are aligned with one another on their flat sides and are stitched together by means of a binding thread, with their folds forming the spine of the book block. The book block has at least two spaced transverse seams formed of a double strand of binding thread that passes through the folds of the signatures from the outside and extends along the inner edge of the fold to their 15 exit points. One of the two transverse seams is formed of a chain of loop-like binding thread sections that are successively pulled out of the folds of the signatures, with the chain being interrupted after a stack section has been formed that determines the book block.
EP-A-0,295,220 discloses an apparatus of the above formulated type according to which book blocks are separated from a stack of signatures by cutting through the binding thread at the so-called flat seam under the influence of a tensioning force generated by the moving stack of signatures. These threads are pulled out of the fold of the signatures at the adjacent loop seam or chained seam, respectively, and then cut apart by a knife. This manner of proceeding produces a breather or relaxation, respectively, between the signatures which has a negative influence on the stability of the book blocks and the compactness to be produced by the stitching.
By changing the binding thread from the hook needle to the knife, an additional unreliability moment is created with respect to the transfer.